ECG Reference Notes

Standard ECG Calibration
  •  Paper speed = 25 mm/s
  • One large square = 0.2s
  • One small square = 0.04s
  • Amplitude = 10 mm/mV

Rate Calculation
  • Assuming standard paper speed there are several methods to calculate the heart rate.
  • Count number of large squares between consecutive R waves and divide into 300.
  • Count number of small squares between consecutive R waves and divide into 1500.
  • An ECG recoding captures10 seconds of activity, so you can multiple the number of complexes along the rhythm strip by 6 to estimate the rate, a useful method for irregular rhythms.

Normal Ranges
  • PR Interval: 0.12 - 0.2s (3-5 small squares)
  • QRS Duration: < 0.12s (3 small squares)
  • QTc usually less than half the R-R interval.
  • Consider prolonged QTc either > 440ms or > 460ms for males and >470ms for females.
  • Considered shortened QTc if < 330ms


  • Normal: -30 degress to 90 degrees
  • Left axis deviation: < -30 degrees
  • Right axis deviation: > 90 degrees
  • Extreme Axis: 180 degrees to -90 degrees

Hex Axial

Image taken from

QRS Nomenclature Examples

The QRS complex represents ventricular depolarisation, however, describing the QRS complex with these three letters is often an oversimplification. A QRS complex may not necessarily contain a Q wave, a R wave, or a S wave, and may contain more than one R wave.

The various deflections of the QRS wave can be notarised as follows:

             Q         q          R          r           S          s           R’         r’

Capitalisation of the letter indicates tall or deep waves, with small waves notarised with small letters.

Directionality and order also determines nomenclature, as below:
  • Q wave – any initial negative deflection
  • R wave – any positive deflection
  • S wave – any negative deflection after a R wave
  • R’ wave – any positive deflection that follows an S wave
  • QS complex – if there is only a single negative deflection
Examples of various QRS complex morphologies and there nomenclatures below


  •  Chan T, Brady W, Harrigan R, Ornato J, Rosen P (2005), ECG in Emergency Medicine and Acute Care, 1st Edition, Elsevier Mosby.
  • Hampton J (2003), The ECG in Practice, 4thEdition, Churchill Livingstone.
  • Surawicz B, Knilans T (2008), Chou’s Electrocardiography in Clinical Practice, 6th Edition, Saunders Elsevier.
  • Morris F, Edhouse J, Brady W, Camm J (2003), ABC of Clinical Electrocardiography, 1st Edition, BMJ Books.